The present invention, in some embodiments thereof, relates to applied materials and more particularly, but not exclusively, to novel dopants of organic semiconductors.
Organic semiconductors (OSCs) are taking an ever growing part in the field of advanced electronics. OSCs are organic materials that have semiconductor properties, namely an electrical conductivity between that of typical metals and that of insulating compounds. Organic semiconductors can take the form of single molecules, short chain oligomers and long chain polymers, such as aromatic hydrocarbons, which include pentacene, anthracene and rubrene as semiconductive small molecules, and poly(3-hexylthiophene), poly(p-phenylene vinylene), F8BT, polyacetylene and derivatives thereof as semiconductor oligomers and polymers.
The electron carriers in polymeric OSCs (organic semiconductors in the form of oligomers and polymers) include π-electrons and unpaired electrons, which allow electrons to dislocate via π-electron cloud overlaps, while in charge transfer complexes, quasi-stable unpaired electrons are the charge carriers. Another charge transfer mechanism in OSC is also obtained by pairing an electron donor molecule with an electron acceptor molecule. According to the terms used in the field, a current is generated by the movement of an electron (denoted “n” for negative) or a “hole” (denoted “p” for positive). The presence of electrons or holes, which are termed n-type or p-type semiconductor material, respectively, is the basis for any conductivity of a semiconductor. Junctions between regions of n- and p-type semiconductors create electric fields or electronic-band offsets which are essential for a range of semiconductor-based electronic devices.
The intrinsic electrical properties of semiconductors can be augmented and adjusted by introducing chemical impurities thereto; a process known as doping.
Unlike some occurrences in the literature of the term “doping” in the context of semiconductors, that use it to denote mixing of small amounts of one substance into a bulk of another substance without effecting a specific augmentation of electrical conductivity but rather effect color, morphology, ion transfer and other physicochemical phenomena, the term “doping”, as used herein and is known in the art, exclusively refers to the protocol of electrical doping where the doping results in the enhancement of charge carrier density in the doped semiconductor material, as oppose to general “mixing” where no electrical doping occurs.
Dopants can be added to preparations of semiconductor substances so as to modify their electrical conductivity. Addition of dopant may result, in some cases, in OSCs that exhibit electrical conductivity nearly as some metals. Depending on the kind of dopant, a doped region of a semiconductor is altered in the number of electrons or holes. The term “n+” is used for n-type dopant, and the term “p+” is used for p-type dopant. Density differences in the amount of impurities also produce small electric fields in the region which is used to accelerate non-equilibrium electrons or holes.
The electrical conductivity of organic semiconductors is strongly influenced by doping. Organic semiconductor matrix materials may be made up either of compounds with good electron-donor properties or of compounds with good electron-acceptor properties. Thus, doping of any semiconductor, and particularly doping of OSCs, has an effect on the electronic performance of a semiconductor primarily by elevating the charge carrier density and hence also, in some cases, the effective charge carrier mobility.
In addition to permanent modification through doping, the conductivity/resistance of semiconductors can be modified dynamically by applying electric fields and other sources of external energy such as electromagnetic energy (light), thermal energy (heat) and magnetism. The ability to control resistance/conductivity in regions of semiconductor material dynamically through the application of external energy sources is probably one of the main applicable features of semiconductors. This capacity has led to the development of a broad range of semiconductor devices such as transistors and diodes. Semiconductor devices that have dynamically controllable conductivity, such as transistors, are the building blocks of integrated circuits devices like the microprocessor. These “active” semiconductor devices (transistors) are combined with passive components implemented from semiconductor material such as capacitors and resistors, to produce complete electronic circuits.
The control of conductivity via n- and p-type doping has been proving important in the realization of low voltage and efficient organic light-emitting diodes (OLEDs). Investigations of high quality and stable electrical doping have focused predominantly on small π-conjugated molecules by vapor diffusion and deposition with limited attention drawn to doping of solution processed conjugated polymers. One of the first reports of useful intentional p-doping made use of fluorinated TCNQ (tetrafluorotetracyano-quinodimethan, F-4-TCNQ) [1] which ever since seems to be the only π-conjugated p-dopant to be used commercially [2].
Several studies aimed at expanding the field of dopants for OSCs and methods of doping OSCs have been published and taught in, for example, U.S. Pat. Nos. 6,908,783 and 7,151,007, and U.S. Patent Application Nos. 20050040390, 20050061232, 20050121667, 20050139810, 20070145355, 20070215863 and 20070278479.
By electron transfer processes, strong electron acceptors such as tetracyanoquinonedimethane (TCNQ) or 2,3,5,6-tetrafluorotetracyano-1,4-benzoquinonedimethane (F-4-TCNQ, see Scheme 1 below) have become well known [1; 3] to produce so-called holes in electron donor-like base materials (hole-transport materials), owing to the number and mobility of which the conductivity of the base material is relatively significantly varied. For example, N,N′-perarylated benzidines TPD or N,N′,N″-perarylated starburst compounds, including 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (TDATA, see, Scheme 1) and certain metal phthalocyanines, such as zinc phthalocyanine (ZnPc), are known as matrix materials with hole-transport properties.

The compounds previously investigated have disadvantages for technical use in the production of doped semiconductor organic layers or of suitable electronic components with doped layers of this kind, since the manufacturing processes in large technical production plants, or those on a technical scale, cannot always be sufficiently precisely controlled. This leads to high control and regulation expense within the process in order to obtain the desired product quality, or to undesirable tolerances of the products.
In addition, there are disadvantages in the use of previously known organic donors with regard to electronic component structures such as light-emitting diodes (OLEDs), field-effect transistors (FETs) or solar cells themselves, since the said production difficulties in the handling of dopants may result in undesirable heterogeneities in the electronic components or undesirable aging effects of the electronic components.
At the same time, however, care has to be taken to see that the dopants to be used have appropriate electron affinities and other properties suitable for the particular application, since, for example, under certain conditions the dopants also help to determine the conductivity or other electrical properties of the organic semiconductor layer.
Fullerenes and derivatives thereof have been used in the context of semiconductors and OSCs both as a substance for the OSC matrix as well as for doping surfaces of inorganic semiconductors.
For example, U.S. Pat. No. 7,358,538 teaches organic light-emitting devices with multiple hole injection layers containing fullerene as the OSC matrix, wherein the layered structures include a bi-layered structure including an electrically conductive layer serving as electrical contact to external circuit and a fullerene layer sandwiched between the conductive layer and a hole transport layer.
U.S. Patent Application No. 20070278479 teaches n-doping of organic semiconductors, wherein fullerenes constitute the OSC material which is then n-doped by organometallic dopants such as bis(2,2′-terpyridine)ruthenium.
Wöbkenberg et al. [4] teach fluorine-containing C60 fullerene derivatives, used as an OSC matrix for high-performance electron transporting field-effect transistors and integrated circuits.
Liming Dai et al. [5] teach the use of C60 spherical fullerenes as carriers for (sulfonated) molecules which are used for p-doping of conducting polymers, namely produce a dendrimer-like molecular structure wherein the fullerene provides the center core of the dendrimer and thus acts as a physical carrier for the electrically active sulfonic molecules.
U.S. Pat. No. 7,371,479 teaches a method for producing fullerene derivatives, similar to those described by Liming Dai et al., from halogenated fullerenes as starting materials, which can be used as proton (positively charged hydrogen atoms or H+) conductors and hence can be used in electrochemical devices.
Sque et al. [6] teach a doping method and semiconductor devices using the same, wherein fullerene derivatives are attached to the top surface of inorganic semiconductors layers in order to induce charge transfer from the molecule to the semiconductor top surface.
Rincóna et al. [7] report the electrical and optical properties of molecular films made of homogeneous and segregated mixtures of polythiophene (PT) with C60 and C60(OH)24-28 compounds. This paper shows the importance of C60(OH)24-28 as a buffer layer between PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), a polymer mixture of two ionomers) and C60-based films for enabling high quality films which are required for efficient solar cell operation.
Sariciftci et al. [8] teach the use of charge transfer in the excited state between semiconductor polymers and fullerenes in their mixed films and their use in solar cell applications.